Osteogenesis Imperfecta (Ol) is a genetic disease caused by mutations in the type I collagen genes COL1A1 or COL1A2 that can result in major skeletal abnormalities, fractures, and premature death. Severe forms of Ol are typically caused by dominant mutations that disrupt the collagen triple helix, so an effective treatment will require the elimination of dominant, mutant alleles. The long-term objective of this proposal is to develop a novel approach for the treatment of Ol based on the transplantation of genetically modified autologous mesenchymal stem cells (MSCs) that produce bone-forming osteoblasts in vivo. These MSCs will be genetically modified by vectors based on adeno-associated virus (AAV) that can efficiently target homologous, chromosomal genes and thereby disrupt mutant collagen genes. In Ol MSCs with mutations in COL1A1, an AAV gene targeting vector disrupted the COL1A1 gene in up to 90% of selected MSCs, which then produced normal collagen and formed bone, in the only published example to date of successful gene targeting in adult human stem cells. Here these findings will be extended by targeting the COL1A2 gene in Ol MSCs with novel AAV vectors that do not encode foreign antigens, and demonstrating that these targeted cells form normal collagen and bone. Targeting frequencies will be determined in MSCs from multiple individuals to evaluate the effects of genetic variation on gene targeting and recombination. Transplantation methods for gene-targeted MSCs will be studied in a rabbit model to optimize long-term MSC engraftment rates and determine safety. These experiments are intended to develop an AAV targeting vector and MSC transplantation protocol that would be appropriate for clinical trials of Ol. These experiments are significant because they will develop a new and promising approach for the treatment of Ol. The research will also have broader implications, since gene targeting methods for human MSCs could potentially be applied to many diseases of bone, cartilage, muscle, and possibly other tissues.